Whereas it is relatively easy to inspect the outer surfaces of tubes, conduits, and the like, inspecting the interior surfaces of these objects poses certain challenges. A first challenge is illumination. Imaging a surface requires that light be directed or otherwise delivered onto the surface. Especially if the tube is made of an opaque material or is surrounded by an opaque material such that light cannot penetrate through the material into the lumen of the tube, delivering sufficient light to the interior surface can be very difficult, especially if the tube has any significant length and/or has light-obstructing internal structure such as ribs or the like. Even if imaging light is deliverable to the inside of the tube, a second challenge pertains to maximizing image-producing light while minimizing image-degrading light. A third challenge is the spatial constraints imposed by the tube geometry. Whatever must be inserted into a tube to produce images of the interior surfaces of the tube must have a smaller diameter than the tube and must be deliverable to locations of interest in the tube. A fourth challenge is keeping image-forming components (e.g., optical elements and camera) of an imaging device clean, particularly in a manner that does not interfere with illuminating and imaging. A fifth challenge is forming good-quality images in which imaging artifacts are minimal and in which tube-structural anomalies of interest are distinguishable. A sixth challenge pertains to determining the locations of anomalies or other features of interest or concern that were discovered during imaging.
U.S. Pat. No. 7,636,204 to Bourget discusses a device for imaging the interior surface of a shallow cylindrical object or hole. The device, essentially a microscope, has a distal end having a source that produces illumination light and a proximal end that includes imaging optics. At the distal end, a compound conical-surface reflector directs light from the source radially onto the interior surface. Light reflected from the surface to the device is reflected from the compound conical-surface reflector to the optics in the proximal end that produce the image. Only the distal end is inserted into the cylinder; the proximal end remains outside the cylinder. One disadvantage of the Bourget system is its inability for use in imaging the interior surfaces of anything other than shallow cylindrical holes. There is no provision for inserting the Bourget system into a hole, tube, or the like having an axial length that is substantially greater than the diameter of the hole. Other disadvantages are the device's vulnerability to incursion of soil and other contaminants from the environment during use, and the difficulty of cleaning the device.
Another device for producing images of the interior surface of a pipe is discussed in U.S. Pat. No. 4,934,813 to Yaginuma. The Yaginuma device has several disadvantages. For example, imaging light is produced at the proximal end of the device, which is not inserted into the pipe; only the distal end is inserted. The imaging light propagates (e.g., by optical cable) in an axial image-transmitting path to the distal end of the device to illuminate the interior surface of the pipe. Any images thus formed must return from the distal end to a camera located at the proximal end. The resulting long passages from the distal end to the proximal end degrades image quality. Second, the Yaginuma device is difficult to keep clean during use, and debris accumulated on the optical surfaces of the device substantially degrades image quality. Third, despite its touted ability to prevent image halation, image quality is not ideal, particularly whenever the Yaginuma device is being used to image an interior surface that is rough, corrugated, or otherwise uneven.